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    <title>Document</title>
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    <script>
        // 预解析案例
        // 案例1
        // var age;
        /* function cook() {
            console.log(age);
        }
        cook();
        var age = 10; */ //位置的不同，输出不同

        // 案例2
       /*  var num = 10;

        function fn() {
            console.log(num);
            var num = 20;
            console.log(num);
        }
        fn(); */

        // 案例3
        var a = 18;
        f1();

        function f1() {
            var b = 9;
            console.log(a);
            console.log(b);
            var a = '123';
        }
    </script>
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